In the field of switch devices, for example, microswitches are widely used in which a switching mechanism is embedded in a switch case, and a movable piece in the switching mechanism is configured to move in response to sliding displacement of a push button serving as an actuator, so that a snap-action movement is effected in order to switch on or off the conduction between terminals (for example, see Patent Document 1—Japanese Laid-Open Patent Publication No. 2006-351391).
Some of the microswitches may be used in an environment where silicone-based materials which generate a siloxane gas are present. Materials such as a silicone-based adhesive or silicone grease, may be used in the vicinity of the microswitch or within the enclosed space where the microswitch is located. If the siloxane gas enters into a case of the microswitch, the gas can cause insulating materials such as SiO2 to be deposited on a surface of a contact provided in the switching mechanism, thus resulting in contact failure.
In order to prevent the deposition of insulating materials, several attempts have been made to create a sealed structure of a microswitch, for example, by sealing connection part of a switch case and covering sliding part of a push button with a rubber cap, or covering the part with a gasket.
For a rubber cap or a gasket, a silicone rubber is frequently used. When a silicone rubber is employed to form a rubber cap or a gasket, the silicone rubber is usually subjected to “secondary vulcanization.” This is carried out for the purpose of preventing deposition of insulating materials from occurring due to the siloxane gas remaining even after molding of the rubber. The secondary vulcanization is useful to remove the residual siloxane gas, but makes the product production or quality control cumbersome.
Another problem in use of a silicone rubber is that, even if the silicone rubber is subjected to the secondary vulcanization, the silicone rubber has a chemical property of being rough in its intermolecular bond, and thus allowing gases to easily pass through.
On the other hand, Patent Document 2 (Japanese Laid-Open Patent Publication No. 2007-042359) discloses a microswitch in which a gasket made of an ethylene/propylene-based copolymer rubber covers the outside of an entire operation button, which corresponds to the push button, to create a sealed state. Patent Document 2 describes that: an ethylene/propylene-based copolymer rubber is used as a material for the gasket; and this makes it possible to prevent the entry of dust, liquids, or gases into the inside of the switch through a coupled portion between the gasket and the operation button from occurring due to up-and-down motion of the operation button, which corresponds to the push button, at the time of actuating the switch. Patent Document 2 also describes that the use of an ethylene/propylene-based copolymer rubber makes it possible to omit the secondary vulcanization procedure for removing the residual siloxane component, and thus facilitating the production of the gasket.
The ethylene/propylene-based copolymer rubber used in the gasket of Patent Document 2, however, has the property of swelling by absorbing grease or other oils. Grease, which is a lubricant, is commonly used for an electronic component, particularly in a switching mechanism, or a plunger portion enabling the sliding displacement of a push button, etc. Further, some switch devices are mounted in automobiles or the like, and various oils are used in automobiles. Thus, there is a problem with the use of a switch device having a gasket made of an ethylene/propylene-based copolymer rubber which is poor in oil resistance, because such a gasket will be deteriorated due to oil adhesion, and as a result, the sealed structured of the device is not maintained in the long-term.